1. Field of the Invention
The present invention relates to a key update method and key update apparatus for updating a key of members in a group. More particularly, the present invention relates to a key update method and a key update apparatus for updating a key of a member which has switched from an offline mode to an online mode.
2. Description of Related Art
Contents which are provided to group members are typically encrypted, so that other users can not use the contents. Accordingly, all of the group members should have an encryption key, provided from a server, capable of decoding the encrypted contents.
Accordingly, updating an encryption key is an important issue. For example, when a new member joins a group, the group is required to allow the new member to restrictively access contents. Later, after the new member joins the group, when the existing group members update the encryption key, the new member may share the updated key.
Also, when one of the existing group members leaves the group, the group key, used by all group members before the leaving member leaves the group, is required to be updated so that the leaving member no longer has access to the contents.
An important issue arises with respect to providing update data, which occurs while a group member device is in an offline mode, when a member switches from the offline mode to an online mode.
Specifically, a device A is required to be able to perform a key update, which occurs during the offline mode, by effectively providing the A device with information of a key update, which occurs during the offline mode in the group in the case that the key update is performed because a new member joins the group or leaves the group while the device A is in the offline mode, and subsequently switches back from the offline mode to an online mode.
For example, in the case that two members join the group, and five members leave the group while the device A is offline, and the device A switches from the offline mode to the online mode, a key update is required to be immediately performed with respect to the two joined members and the five leaving members, occurring while the device A was in the offline mode.
Particularly, an effective key update with respect to a member switched from the offline mode to the online mode becomes an important issue as a number of group members increases, as the key update is frequently performed, and as a length of time being in the offline mode increases.
Accordingly, there is a need for an improved effective key update method and apparatus using the method are required with respect to the member switched from the offline mode the online mode.